1. Field of the Invention
The present invention generally relates to the art of electrophotography and, more particularly, to a multicolor image forming apparatus, such as, for example, a multicolor printer or a multicolor copying machine, of a type operable according to an electrophotographic two-color image forming process.
2. Description of the Related Art
An electrophotographic two-color image forming apparatus has hitherto been proposed which comprises first and second developing units disposed in the vicinity of a photoreceptor for accommodating first and second toner materials of different colors, respectively, so that a first electrostatic latent image formed on the photoreceptor can be developed with the first toner material into a first toner image and a second electrostatic latent image formed on the photoreceptor can be developed with the second toner material into a second toner image, the first and second toner images being subsequently transferred at a time onto a transfer medium to provide a two-color image.
It has however been found that the above described two-color image forming method has a problem in that the first toner image formed with the first toner material may, during a passage thereof past the second developing unit, contact and then mix into a second developing material to such an extent that, with an increase of the number of prints being made, the second developing material becomes impure and the continued use of the impure second developing material will eventually result in that the image formed with the second developing material will show a mixed color. This problem is particularly considerable where the second developing unit is of a magnetic brush type. Considering that the magnetic brush developing unit is of a design capable of accomplishing a development by causing magnetic brush bristles, made of toner particles and carrier particles, to contact the photoreceptor, the use of the magnetic brush developing unit for the second developing unit will result in that the first toner material may be physically removed by the magnetic brush bristles from the photoreceptor.
In view of the foregoing, in order to separate and recover the first toner material from the second developing material, a method has been suggested in, for example, U.S. Pat. No. 4,822,702 issued Apr. 18, 1989, wherein the first toner material mixed into the second toner material is reversed to a polarity opposite to that of charges on the second toner material so that the first toner material can be electrostatically separated from the second toner material. Another method has also been suggested in, for example, the Japanese Laid-open Patent Publication No. 58-102251, published Jun. 17, 1983, wherein the use has been made of the first and second toner materials having different developing threshold values and, in other words, a difference is given to a development initiating potential of the toner material, while a recovery roll to which a bias voltage is applied is used to separate the first toner material from the second toner material.
According to the foregoing suggested separating methods, while it is theoretically possible to separate only the first toner material from the second developing material, a complete separation is not possible since in practice the first toner material being recovered contains a substantial amount of the second toner material. Because of this, when the first toner material is separated and recovered, the second toner material is also recovered, resulting in an unnecessary waste of the second toner material.
While this problem may to a certain extent eliminated if stringent conditions are employed when the first toner material is to be separated, the employment of the stringent conditions may bring about a limitation to the type and the range of the toner material that can be employed or may not accommodate a change in chargeability of the toner material which would occur as a result of a change in environment. Therefore, the employment of the stringent conditions for the separation of the first toner material cannot be regarded as an effective means to accomplish a satisfactory separation of the first toner material from the second toner material.